1. Field of the Invention
This invention relates generally to incinerators, and more particularly to incinerators for the combustion of heterogenous waste materials such as household and commercial refuse. In the embodiments described, it teaches the provision of a removable noncombustible granular substrate beneath the material being consumed in the primary combustion chamber/gasifier of a rotating auger incinerator to aid in the more complete diffusion of combustion air through the materials being burned, to facilitate treatment of undesirable emissions from the materials being burned, and most importantly, to act as an xe2x80x9cair bearingxe2x80x9d for the material being consumed and to thereby facilitate its movement through said chamber. It also teaches the provision of an auger with expandable flights in such an incinerator, the suspension of treatment particles in the gas stream leaving said combustor/gasifier, and numerous other improvements.
2. Prior Art in the Field
Much of the world""s energy needs have been, and continue to be, filled by hydrocarbon fuels. In the past, such fuels provided a convenient, plentiful, and inexpensive energy source. The current rising costs of such fuels and concerns over the adequacy of their supply in the future has made them a less desirable energy source and has led to an intense investigation of alternative sources of energy. The ideal alternative energy source is a fuel which is renewable, inexpensive, and plentiful, with examples of such fuels being the byproducts of wood, pulp, and paper mills, and household and commercial refuse.
The use of such alternative energy sources is not problem-free, however, since there is reason for concern over the contents of the emissions from the combustion of such fuels as well as the environmental ramifications of acquiring and transporting the fuel and disposing of the residue of combustion. Starved-air combustors, wherein the air supplied for combustion is controlled in order to control temperature conditions (and the rates of combustion) so as to consume the fuel entirely, have proved very useful in the utilization of such alternative energy sources while simultaneously maintaining a high degree of environmental quality in emissions. Such starved-air combustors are capable of burning various types of fuel and producing significant amounts of heat which can be employed for any number of purposes including the production of process steam for use in manufacturing and in the generation of electricity.
Unfortunately, most starved-air combustors, as originally developed and operated, were not entirely satisfactory in consuming the combustible elements of the fuel at high throughput while not producing noxious emissions. This problem resulted, in part, from the use of such combustors to burn a wide variety of fuels, including many which were nonhomogeneous, such as household or commercial refuse. While the pollution problem can be solved to a degree by the utilization of scrubbers and other antipollution devices, such mechanisms are very expensive and their cost may militate against the use of alternative energy sources previously described.
Many of the drawbacks of such prior art devices were overcome by the development of the auger combustor/gasifier by the inventor and others. See, U.S. Pat. No. 4,009,667 (describing the original auger combustor/gasifier utilized in the system); U.S. Pat. No. 4,315,468 (describing an incinerator control means for the system); U.S. Pat. No. 4,331,084 (describing a refuse fuel feed mechanism for the system); U.S. Pat. No. 4,331,085 (describing a flame stabilization means for the system); U.S. Pat. No. 4,332,206 (describing an afterburner for the system); and U.S. Pat. No. 4,332,206 (describing a hot gas recycle mechanism for use with the system). The auger combustion technology taught and described in the foregoing patents offers a cost-effective approach to clean, efficient combustion of prepared solid waste and other solid fuels. It employs a starved-air combustion technique, partially combusting or gasifying solid fuel in a primary chamber (the xe2x80x9ccombustor/gasifierxe2x80x9d), then passing the combustible gases to an afterburner where sufficient air is added to complete combustion.
One of the unique features of the auger combustor/gasifier system is the variable-pitch auger. The fuel enters the combustor/gasifier at a controlled rate and is shaped into a pile by the first auger flight. It is then pushed and tumbled through the combustor/gasifier chamber by the auger. As the auger moves the fuel through this horizontal cylinder, it stirs the material to maximize exposure to the oxidizing air injected into the chamber. The pitch of the auger decreases along the path of material flow to accommodate the decrease of fuel bulk as the material combusts. This ability to manage fuel-bed configuration permits control of forced-draft combustion air to gasify nearly all the fuel without complete combustion taking place, thereby allowing the combustor/gasifier to operate at what is, for an incinerator, a uniformly moderate temperature (e.g. 1,500-1800F).
The combination of fuel bed stirring and air injection with precise temperature control gives the auger combustor/gasifier system several advantages over prior technology: Reliability and clean operation; high throughput; low gasifier temperature, afterburner combustion of only gaseous fuels, precise flame structure and temperature control; longer material life (refractory and auger); fully automatic control; and the ability to combust a wide variety of heterogenous solid fuels. However, it has been discovered that several of these advantages may be further strengthened by the addition of a substrate of appropriate granular materials to act as an xe2x80x9cair bearingxe2x80x9d in the auger combustor/gasifier chamber, to aid in the diffusion of combustion air through the material being burned, and to facilitate treatment of harmful emissions from the materials being burned. Moreover, the advantages inherent in the use of such a substrate are even further magnified by its use in conjunction with a combustor/gasifier chamber wherein the pressure of the air forced through the substrate is at its maximum at the fuel input end of the chamber and at its minimum at the output end of the chamber.
Granular fuels being burned in incinerators have been, in the past, formed into fluidized beds via the insertion from underneath of combustion air at appropriate pressures. In such incinerators, the granular fuel forming the fluidized bed is moved by the fluidization means or by a movable grate under the bed. The fluidization of the granular fuel aids in its combustion and its movement through the combustion chamber. However, the inventor is aware of no incinerators in which a fluidized bed acts as a substrate for the material being burned and is formed from a noncombustible material which remains generally stationary with respect to such material, acts as an xe2x80x9cair bearingxe2x80x9d for such material, aids in the diffusion of combustion air into and through such material, and may be utilized to help treat and eliminate undesirable emissions from the material being burned.
In addition to those patents enumerated above with regard to the auger combustor/gasifier, representative patents illustrating the current state of the art in the area of starved air incinerators include the following:
1. xe2x80x9cApparatus for the Combustion of Poorly Combustible Fuelsxe2x80x9d issued to Cosar (U.S. Pat. No. 4,809,620) in 1989.
2. xe2x80x9cMethod and Apparatus for Regulating the Furnace Output of Incineration Plantsxe2x80x9d issued to Martin (U.S. Pat. No. 4,953,477 in 1990.
3. xe2x80x9cMethod and Apparatus for the Efficient Combustion of a Mass Fuelxe2x80x9d issued to Barlow (U.S. Pat. No. 5,044,288) in 1991.
4. xe2x80x9cIncinerating-Fusing System for City Refuse Disposalxe2x80x9d issued to Tsunemi et al. (U.S. Pat. No. 5,078,065 in 1992.
5. xe2x80x9cProcess and Apparatus for Emissions Reduction from Waste Incinerationxe2x80x9d issued to Khinkis et al. (U.S. Pat. No. 5,205,227 in 1993.
Upon review of these patents it will be found that none anticipate or render obvious the unique innovations described herein.
The efficient and trouble-free operation of the system as a whole depends in part on the reliability of its most distinctive feature-the auger (with its water cooled hollow shaft) which is utilized to convey fuel through the combustor/gasifier. In this regard, it has been discovered that (in operation) the auger is more readily subject to failure if provision is not made for the expansion of the material making up its flights when subjected to elevated temperatures within the combustor/gasifier chamber.
Numerous means have been utilized in the attempt to create auger conveyors that can operate without problems at high (and possibly varying) temperatures without failure. Representative patents in this area include the following:
1. xe2x80x9cExpansion Coupling for Screw Conveyorsxe2x80x9d issued to Parker (U.S. Pat. No. 1,829,453) in 1931.
2. xe2x80x9cFloating Auger Flight for Conveyorsxe2x80x9d issued to Mayrath (U.S. Pat. No. 3,219,178) in 1965.
3. xe2x80x9cScrew Conveyor Apparatusxe2x80x9d issued to Christian (U.S. Pat. No. 3,637,069) in 1972.
4. xe2x80x9cTemperature Expansive Screw Conveyorxe2x80x9d issued to Millsap (U.S. Pat. No. 4,399,906) in 1983.
However, none of these means is suitable for use with the unique auger conveyor utilized in the auger combustor/gasifier system previously described.
U.S. Pat. No. 1,829,453 issued to J. N. Parker in 1931 is appropriately titled xe2x80x9cExpansion Coupling for Screw Conveyors,xe2x80x9d and teaches the design for an auger featuring such a coupling in its shaft. However, shaft expansion is not a problem in the instant system as the shaft is water cooled. The problem is, instead, finding a means of maintaining the distinctive configuration of the auger flights utilized in the instant system when the system is in operation (and therefore subject to the elevated temperatures within the combustor/gasifier) despite the expansion of the material making up the auger flights.
U.S. Pat. Nos. 3,219,178 and 3,637,069 issued to Mayrath and Christian, respectively, likewise fail to solve this problem. Both teach systems wherein the flights of the auger are, in general, slidable with respect to the shaft. Thus, the flights are enabled to expand and lengthen when exposed to elevated temperatures, but only by sacrificing the integrity of the auger""s initial configuration. This is totally unacceptable in the context of the instant system as the configuration of the auger conveyor utilized, with its tapering flights, is an integral part of the system""s design and must be maintained in order to guarantee the proper functioning of the system.
U.S. Pat. No. 4,399,906 issued to Millsap teaches the design for an expandable auger wherein the auger flights are attached to stakes that extend completely through the shaft and are slidable with respect thereto. Thus, as the material making up the auger flights expands, the flights are enabled to expand away from the shaft, increasing the overall diameter of the auger. This is also unsuitable in the current application. The auger flights of the auger utilized in the current application must be maintained at precise distances from the floor and sides of the combustor/gasifier. These relationships would be distorted if the diameter of the auger was allowed to expand. Further, it would be unsuitable in the current application because of the openings in the shaft that allow the penetration of the stakes therethrough. This would, in the instant application, interfere with or render impossible the water cooling of the shaft. Thus, there are no solutions provided in current or preexisting art for the flight expansion problem required to be solved for the optimum functioning of the auger utilized in the auger combustor at elevated temperatures.
The inventor is aware of no prior art similar to that taught herein in the field of waste incineration. However, the techniques for the suspension of particulates in a vertical gas stream are well known in the gypsum industry where vertical driers are used in the manufacturing of gypsum board.
The use of addition fuel sources to assist in the firing and combustion of less combustible materials, such as the input of an inflammable gas stream (e.g.-propane), is well known in the field of waste incineration. The inventor is, however, not aware of any prior use of such techniques in a device of the type described herein.
It is desirable in waste incineration to increase the residency time in the incinerator of various gases released by the incineration process. The volumetric increase of a gas channel as a means of slowing the flow of gas therethrough is well understood in the arts pertaining to fluid dynamics. The inventor is, however, not aware of any prior use of such techniques in a device of the type described herein.
The auger combustor/gasifier constructed in accordance with the teachings of this invention features, as is typical for incinerators of this type, an auger that is (a) disposed in a horizontal position within the combustor/gasifier chamber in close proximity to the bottom of the chamber such that it is proximate the (b) the refractory material that covers the interior of the chamber.
In its most basic embodiment, the instant invention would add a layer of noncombustible granular material to the bottom of the combustor/gasifier chamber (between the auger and the bottom of the chamber), which noncombustible granular material is suspended and maintained in a substantially fluidized state by the influx/insertion of air at high pressures from below and serves as a supporting substrate for the materials being incinerated in the chamber.
The addition of this layer of noncombustible granular material in the form of a fluidized fuel supporting substrate has, as previously noted, several important benefits. First, it acts to provide the equivalent of an xe2x80x9cair bearingxe2x80x9d as the granular materials utilized for the substrate are substantially suspended or floating in the high pressure air inserted from underneath. It has been found that this substantially lowers the amount of effort/energy required (via the auger) to convey material being burned through the chamber. Second, it acts to further diffuse the air being fed from underneath the fuel burned, assisting the process of complete incineration. Third, it provides a buffer or pad between the heterogenous and often abrasive materials being incinerated and the refractory, protecting the refractory from abrasion or other damage and prolonging its useful life. Fourth, by utilizing limestone or other appropriate treatment materials for the granular substrate, this system provides a means of treating gaseous emissions from the materials incinerated, avoiding or reducing the need for other treatment means which absorb useful heat and thereby reduce profitability.
In its more advanced and preferred embodiment, as illustrated herein, the auger combustor/gasifier constructed in accordance with the teachings of this invention also features two other improvements and innovations. First, the granular material forming the substrate is readily renewable, being readily placed and removed. Second, the pressure of air injected from underneath the granular material decreases between the fuel input end of the combustor/gasifier chamber and the output end thereof.
Renewability of the granular material utilized for the substrate is accomplished by inclusion of means for the ready insertion of said granular material and construction of the incinerator in such manner that the auger, which is typically suspended and held in position at opposite ends of the chamber, may be readily raised away from the floor of the gasifier chamber or lowered to a position adjacent thereto. The ability to raise and lower the auger allows the placement of the substrate of granular material utilized by this invention on the floor of the chamber (when the auger is elevated) and its ready removal from the chamber (when the auger is lowered so that its flights sweep the material from the chamber). A supplemental auger or other means is utilized to inject limestone or other substrate material into the auger combustor chamber.
While the desirability of providing means for the ready renewability of bed material requires no explanation and is readily understood, the objects accomplished by the injection of the air utilized to hold the granular material in suspension at diminishing pressures along the length of the chamber require reference to the auger combustor/gasifier incineration process. In this process, fuel introduced into the auger combustor/gasifier chamber is consumed/gasified as it is moved through the chamber by the auger, resulting in an ever smaller volume and weight of fuel material as it is moved from the point where it is introduced into the chamber. If air is inserted at a constant pressure from underneath the substrate for the entire length of the chamber, the pressure may be either undesirably low at the end of the chamber where fuel is injected (the xe2x80x9cinput endxe2x80x9d) and the fuel being consumed is heaviest, or undesirably high in the opposite end of the chamber (the xe2x80x9coutput endxe2x80x9d) where most of the fuel has been gasified and the remainder to be consumed is lightest. Thus, supplying air to the chamber at a constant pressure may be either: (a) insufficient to supply necessary oxygen and to create the desired xe2x80x9cair bearingxe2x80x9d effect at the input end of the chamber; or (b) it may be (if pressure is high enough to accomplish these goals at the input end of the chamber) too high in the output end of the chamber, resulting in the lifting of substrate, ash, debris and unburned fuel/refuse from the floor of the chamber and its possible dispersion into other parts of the system with potential disruptive effects. The inventor""s solution to this problem is the subdivision of the auger combustor chamber into segments with each successive segment having its own plenum supplying air at successively diminishing pressures.
As previously noted, the auger gasifier/combustor is distinguished by an auger with a water cooled shaft and flights that are spaced from the shaft and held in position relative to the shaft by xe2x80x9cstandoffsxe2x80x9d which extend outwardly from the shaft like spokes, but do not penetrate the shaft. Due to the extreme heat in the incinerator environment, the material that makes up the flights of the auger expands when the incinerator is in use. If means are not provided to absorb and/or counteract the expansion in each segment of the auger flights, the shape of the auger may become distorted, stress will be placed on the uprights holding the auger flights, and the system may fail or fail to perform in an optimum manner.
The inventor has solved these problems by dividing the flights of the auger into separate sections with each section being provided with leeway to expand only in a manner that does not distort the overall configuration of the auger. In general, each section of the auger flight is fixed to four uprights; however, these numbers may vary. Each such section is slidably attached to all but one of these uprights. Further, where sections meet, they slidably overlap. These design innovations allow the lengthwise expansion of each flight only along the spiral path dictated by the placement of the standoffs without distortion of the configuration of the auger.
The direct suspension of cleaning material in the gas stream leading from the incinerator to the afterburner serves to greatly enhance the cleaning of the gas stream and the removal/treatment of possible pollutants.
The addition of fuel sources and firing mechanisms at the input end of the auger combustor incinerator serves to greatly increase throughput when needed to maintain the pace of treating incoming waste materials and is greatly facilitated by the use of the granular fluidized bed mechanisms described herein.
The volumetric expansion of the auger combustor chamber from input to output end is extremely useful in slowing the flow of gas therethrough, leading to increased residency time in the incinerator and a more complete break-down of various gases produced.